Catalytic reforming of naphtha feedstocks is well known in the petroleum refining industry. Most naphtha feeds contain large quantities of naphthenes and paraffins and consequently they have low octane numbers. In catalytic reforming these components go through a variety of hydrocarbon conversions resulting in a gasoline product of improved octane number. Some of the more important conversion reactions include dehydrogenation of naphthenes to aromatics and dehydrocyclization of normal paraffins to isoparaffins. Less desirable reactions which commonly occur include hydrocracking of paraffins, naphthenes and dealkylation of alkylaromatics to produce gaseous hydrocarbons such as methane and ethane. Because of these less desirable reactions, an important objective of catalytic reforming is to rearrange the structure of the hydrocarbon molecules to form higher octane products without any significant change in the carbon number distribution of the stock.
The reforming reactions are, typically, catalyzed by catalysts comprising porous supports, such as alumina, that have dehydrogenation promoting metal components impregnated or admixed therewith. Platinum on alumina and more recently bimetallics such as platinum and rhenium on alumina are examples of these catalysts. Such catalysts are described in U.S. Pat. Nos. 3,415,737 and 3,953,368.
Other known reforming catalysts have been based on zeolites containing a noble metal component such as platinum. U.S. Pat. No. 4,582,815 describes a silica-bound zeolite catalyst composition for various hydrocarbon conversions including reforming. U.S. Pat. No. 4,839,027 describes a reforming process which employs an intermediate or large pore zeolite bound with a low acidity refractory oxide binder material and containing at least one metal species selected from the platinum group metals. Typically, reforming is operated at pressures below about 350 p.s.i.g. (2,514 kPa) and in the presence of hydrogen.
Procedures for upgrading a reformate to achieve selective rearrangement and increased yields of high octane products have been described in several United States patents. These procedures include selective hydrocracking, see U.S. Pat. No. 3,806,443; low severity hydrocracking, see U.S. Pat. No. 3,847,792; and aromatics alkylation, see U.S. Pat. No. 3,767,568.
Thermally treated zeolites have been described in U.S. Pat. No. 3,923,641 where a high activity zeolite beta catalyst is used in hydrocracking a reformate by heating the catalyst at high temperatures, ranging from 400.degree. F. (204.degree. C.) to 1,700.degree. F. (927.degree. C.) for one to 48 hours to achieve a strongly acidic material. A broad range of hydrocracking conditions are described including temperatures ranging from 400.degree. F. (204.degree. C.) to 600.degree. F. (316.degree. C.) and pressures from 0 to 2,000 psig (101.4 kPa to 13,891 kPa). In U.S. Pat. No. 4,016,218 a process for alkylating aromatic hydrocarbons over a thermally modified crystalline aluminosilicate is described.
Various methods for steam treating zeolites to enhance the properties of the zeolite have been described. Steaming a zeolite to improve the stability during hydrocarbon conversion reactions is disclosed in U.S. Pat. Nos. 4,429,176 and 4,522,929. The zeolite of improved stability is made by mildly presteaming the catalyst under controlled conditions of temperature, time and steam partial pressure. A method for enhancing the activity of a zeolite catalyst by forming the catalyst into a composite with an alumina binder and steaming the composite is described in U.S. Pat. No. 4,559,314.
It is known that benzene, toluene and xylenes can be produced from a reformate feed containing benzene and alkyl aromatics over a zeolite of reduced activity, such as steamed ZSM-5 under high temperature conditions, see U.S. Pat. No. 4,224,141. However, the described conditions also require low pressures, below about 100 psig (791 kPa), preferably lower, and an absence of hydrogen. These conditions are incompatible with the pressure conditions and the presence of hydrogen in the reformer so the feed is not used directly from the reformer. Rather, it is first fractionated and a portion of the effluent is sent to the hydrocracker.
During processes for the production of hydrocarbons employing an acid zeolite catalyst, depletion of catalytic activity occurs. This catalyst deactivation can generally be ascribed to the nature of the feed, the nature of the catalyst itself and/or the processing conditions. More specifically, catalyst deactivation can result from the deposition of organic matter onto the catalyst which is typically referred to as "coking", or from a reduction in the zeolite framework aluminum content. In both instances, it is the acidic function of the zeolite catalyst that becomes diminished or destroyed.
Some catalysts which have become deactivated because of coking can be regenerated by burning in an oxygen-containing gas or removing the organic matter from the zeolite in a hydrogen-containing gas. See U.S. Pat. No. 4,358,395.
Although burning in an oxygen-containing gas and treatment with hydrogen are known to regenerate certain catalysts, these processes in general require high temperature and are costly. Furthermore, the regeneration often fails to fully restore all properties so that the regenerated catalyst is not considered to be the same as a "fresh" catalyst. However, as mentioned earlier, the regeneration is only known to regenerate catalysts which have become deactivated from coking. Such techniques are not recognized as being effective to reactivate a zeolite which has been deactivated because of framework dealuminization.
During certain catalytic conversion processes, such as the methanol-to-gasoline (MTG) process, conditions are such that zeolite framework dealuminization might be expected. For instance, MTG processing is typically conducted at elevated temperatures. Water vapor produced is known to attack aluminum atoms present in the zeolite framework and to remove them in the form of aluminum oxide and/or hydroxide clusters. The loss of framework aluminum is detrimental to these catalysts since catalytic activity is generally attributed to framework aluminum atoms and/or cations associated with aluminum atoms.
U.S. Pat. No. 4,919,790 discloses a method for reactivating a deactivated zeolite catalyst so that the reactivated catalyst may be used for hydrocarbon dewaxing. A method for upgrading a reformate which utilizes a catalyst deactivated by MTG processing is not described.
Recently, it has been reported that pollution can be reduced by lowering gasoline endpoint to result in a product endpoint where, in a standard ASTM distillation, 90 volume percent of the gasoline distills below about 270.degree. F. (132.degree. C.) to 350.degree. F. (177.degree. C.) (T.sub.90). Based on this, there have been regulatory proposals, particularly in the State of California, to require gasoline to meet a maximum T.sub.90 specification of 300.degree. F. (149.degree. C.). Meeting this T.sub.90 permits only 10% of the hydrocarbons in gasoline to boil above 300.degree. F. (149.degree. C.). A significant boiling range conversion of heavy naphthas will be required to meet this goal.